The present invention relates to an improved multistage angle encoder of the type which includes a drive shaft, a fine code disc coupled to the drive shaft, a gear drive coupled to the drive shaft, a coarse code disc coupled to the gear drive, and a scanning unit positioned to scan the fine and coarse code discs.
Such multistage angle encoders serve for the measurement of the angular position of a shaft over several revolutions. Such encoders can be used, for example, in order to determine the absolute position of a slide piece of a machine tool based on the measured angular position of the drive spindle. Two examples of such angle encoders are disclosed in German DE-AS No. 21 26 510, and German patent No. DE-PS 27 46 854.
Such angle encoders, which are designed to measure an angle n.multidot.360.degree., include a first code disc connected with the shaft of the angle encoder, and this first fine code disc clearly defines the absolute fine position of the shaft in z steps within the angle of 360.degree.. In addition, at least one coarse code disc is provided which is connected with the shaft of the angle encoder by means of a reducing gear with a reduction factor n. This coarse code disc is used to characterize the absolute coarse position of the shaft within the angle n.multidot.360.degree. in a clear, unambiguous code. In this way, the absolute position of the shaft is represented in coded form as one of a number of n.multidot.z angular steps.
Preferably, angle encoders utilize one-step codes, in which adjacent binary combinations are distinguished from one another by a change in only a single binary place, in order to define the coded angle in such a way as to reduce undefined intermediate stages during the transition from one angular position to the next angular position. Such one-step codes in general tend to reduce the electronic complexity and cost of the system. In such one-step codes, however, the measuring accuracy is determined by the code information data on all of the code tracks on both code discs. The reducing gears of a multistage angle encoder are, however, in general insufficiently accurate to allocate the code information of the coarse code disc exactly to the code information of the fine code disc. That is, the two code discs can be positioned out of synchronization with respect to one another. For this reason, the code information of each code track of the coarse code disc is typically double-scanned (once with a phase lead and once again with a phase lag). A signal derived from the fine code disc is used in conjunction with a switching logic circuit to determine which coarse code information (the one with phase lead or the one with phase lag) is to be used (principle of U-scanning). This double scanning, however, is relatively expensive to implement, especially in the case of large measuring lengths which result in the use of many code tracks.